(1) Field of the Invention
The present invention relates to an improvement in pneumatic safety tires, and more particularly the invention relates to pneumatic safety tires having improved riding comfortability against vibrations (hereinafter referred to as anti-vibration riding comfortability) as well as improved durability in upper regions of sidewall portions.
(2) Related art statement
Since safety tires can be run flat over some distance even under a reduced inner pressure as is, when the inner pressure of the tire decreases due to trouble such as a puncture, such safety tires have recently come to be used because of their safety performance.
Known structures for the safety tires capable of being run flat include, a double wall structure, a tire interior-supporting coat structure, and a tire sidewall-reinforcing structure as shown in FIG. 2 are known. Among them, a side-reinforced tire having inner surfaces of the sidewall portions reinforced mainly with rubber layers has been put into practical use due to its lower production cost, excellent rim-assembling performance and excellent run-flat running performance.
That is, FIG. 2 is a sectional view illustrating a conventional side-reinforced safety tire. In this conventional safety tire, a pair of sidewall portions 1 extend substantially radially inwardly from opposite ends of a cylindrical tread T, respectively, bead rings 2 are buried in tip end portions of the respective sidewall portions 1, and a carcass 3 consisting of at least one ply having fiber cords arranged is turned up around the respective bead rings 2 from an axially inner side to the axially outer side, and each of turn-up portions 3' of the carcass is formed at a relatively radially low position.
A rubber filler 4 is arranged between the main body and the turn-up portion 3' of the carcass 3. This rubber filler has a triangular sectional shape contacting the bead ring 2 at its base portion, and is made of a relatively hard rubber having, for example, Shore A hardness of 70-98 and a large volume. Further, the tire is reinforced by arranging a thick rubber reinforcement layer on an axially inner side of the carcass 3 in the sidewall portion.
The rubber reinforcement layer is divided substantially in a direction of a rotary axis of the tire (hereinafter referred to as "axial direction"), and is constituted by the relatively soft first reinforcing rubber layer 5 having, for example a Shore A hardness of 60-80 and positioned outwardly in the axial direction and the relatively hard second rubber reinforcing layer 6 having, for example, a Shore A hardness of 75-90 and positioned inwardly in the axial direction.
A relatively soft rubber chaffer 7 having excellent wear resistance is arranged axially outwardly along the turn-up portion 3' of the carcass from a radially inner end portion of the bead portion which contacts a rim when the tire is fitted to the rim. Following the rubber chaffer 7, the sidewall portion is entirely covered with a side rubber 8 having excellent bending resistance.
However, with respect to the conventional safety tire, particularly, a conventional high performance tire, having the above construction, not only high running performance but also excellent living comfortability, particularly, anti-vibration riding comfortability during ordinary running (no run-flat running) are required. However, since the sidewall portions and the bead portions are entirely reinforced with relatively hard rubber as mentioned above, anti-vibration riding comfortability particularly during ordinary running is unfavorably poor.
Since the above-mentioned conventional safety tire largely changes its curvature during run-flat running in the case of puncture, the rubber reinforcing layer is cracked on its inner surface in the upper part of the sidewall portion by synergistic effects of stress concentration, heat generation and heat accumulation. Consequently, such cracks are likely to be propagated to unfavorably cut the carcass.
In particular, since the rubber filler 4 has a relatively large volume and is surrounded by the main body and the turn-up portion of the carcass 3, its rigidity, particularly its bending rigidity becomes extremely high. Consequently, this causes not only stress concentration in the upper part of the sidewall portion but also reduction in anti-vibration riding comfortability.